The dynamic effects of a vehicle passing over a weight sensor at speed are minimised by the weight sensor protruding as little above the road surface as possible. With weight sensors being placed on top of the road surface this requires the weight sensors to be as thin as possible.
The Inventor is aware of weight sensors of this kind which comprise a sandwich construction of three electrically conductive sheets, such as aluminium sheets, separated from each other by elastic strips, such as polyurethane (PU) or silicon strips, bonded between the inner sheet and the two outer sheets. A signal from a stable oscillator is applied between the inner sheet on the one hand and the two outer sheets on the other and a circuit measures the change of capacitance which results when a vehicle passes over the weight sensor. The change in capacitance results from the sheets being compressed together by the weight of a vehicle wheel or wheels on a single axle or axis which may generically be termed an axle set. The change in capacitance is then converted electronically to produce a number related to the weight of the axle.
Such weight sensors have stringent environmental conditions to meet. Temperatures on road surfaces range between sub-zero to over 50 degrees centigrade, the oscillator and other circuit components must remain reasonably stable, the PU elasticity must remain reasonably stable and the bond of the PU to the sheets must remain intact. The sandwich must be impervious to entry of water between the sheets as this immediately causes leakage currents and adversely affects the security of the bond and the stability of the oscillator. Because it is necessary to measure weight of all axle sets of a vehicle which pass over a weight sensor within milliseconds of each other when a vehicle is at speed the PU strips must elastically recover from compression very quickly.
The Inventor is aware that these requirements have been substantially met by a product of this type which is moreover not unduly expensive compared, for example, to load cell technology. However, this type of product is manufactured by a laborious method requiring of the order of one man-week per weight sensor. The aluminium sheets are surface treated or primed for bonding and metal strips, which serve as moulding cores, are prepared with a release agent. A module containing the oscillator and circuitry and the metal strips are assembled between the sheets in a jig. PU is injected into the gaps between the metal strips and allowed to cure over 24 to 48 hours. The metal strips are then pulled out and the edges of the sheets are sealed by PU, applied around the edges, and allowed to cure.
Apart from the laborious nature of this method, quality control in the manufacture of such weight sensors is difficult and tends to be inconsistent. The weather at the time of the work causing greater or lesser humidity, impurities in the air of dust, oils or other workshop pollutants even only in parts per million which are normally not noticed affect the quality of the bonds, as well as errors by the artisans placing the release treated strips, etc. Bond quality is important because the compression of the PU strips causes shear forces at the bond so that any weakness in the bond results in partial or complete failure of the bond in service. Once failed the response characteristics are distorted and the weight sensor becomes unserviceable.
These problems inhibit the development of this industry from being able to supply substantial orders of reliable consistent quality weight sensors to the market place.
Examination of the role played by the air gaps created between the outer sheets and the inner sheet and between each pair of PU strips reveals that despite air being conventionally accepted as providing a good dielectric for the capacitive effect which is used in the product, the primary functional importance of the air gaps is mechanical. They allow the PU strips space in which to expand laterally when compressed between the sheets and so provide the order of elastic constant appropriate in this application. Volumetrically restricted PU is too rigid. Its elastic response to compression between the sheets, which the air gaps allow, is in fact a shear response or Poisson effect.
Furthermore, the Inventor has noted that the top sheet of such weight sensor tends to suffer damage after prolonged use. This is due to border regions of the top sheet resting on border parts of the weight sensor. Accordingly, when a vehicle passes over the weight sensor, the comparative rigidity of the border parts, and the compression of parts of the sheets proximate the PU strips, results in bending of the top sheet at the border region of the top sheet. Repeated bending results in stress fatigue and eventual permanent damage to the weight sensor. This problem inhibits the development of this industry from being able to supply a more durable weight sensor to the market place.